The present invention relates to a coordinate data input device of an electrostatic capacity type capable of detecting the position of a point on a screen touched mainly with a finger through the detection of a change in electrostatic capacity.
Recently, a coordinate data input device of an electrostatic capacity type generally referred to as tablet has been put to practical use. An operator is able to enter the x- and the y-coordinate of a point on the screen of the coordinate data input device by touching the point with a finger.
Referring to FIG. 3, a conventional coordinate data input device has a printed wiring board, not shown, a sensing board 1 using a rectangular dielectric film 4 as a base film and placed on the printed wiring board, and a protective sheet 3 placed on the sensing board 1. The dielectric film of the sensing board 1 is provided on its front surface with a plurality of parallel x-electrodes 5 of a thickness in the range of 7 to 10 .mu.m extended in parallel to the x-axis, and on its back surface with a plurality of parallel y-electrodes, not shown, extended in parallel to the y-axis. An adhesive layer 2 of an acrylic resin or the like is formed on the back surface of the protective sheet 3 to bond the protective sheet adhesively to the sensing board 1. The protective sheet 3 must have an appropriate thickness of about 0.1 mm so that the front surface of the protective sheet 3 serving as a touch surface to which touches are given with a position indicator, such as a finger, is kept flat and the protective sheet 3 is able to protect the x-electrodes of the sensing board 1 from operating force applied to the front surface thereof. A circuitry, not shown, for driving and controlling the electrodes of the sensing board 1 is formed on the back surface of the printed wiring board.
When an operator's finger is put to an optional position on the protective sheet 3 overlying the sensing board 1, part of lines of electric force extending from the x-electrodes toward the y-electrodes is intercepted by the operator's finger to reduce lines of electric force reaching the y-electrodes, so that the electrostatic capacity changes, and the coordinates of the position touched with the operator's finger can be determined on the basis of an output current produced by the sensing sheet varying according to the variation of the electrostatic capacity.
In this conventional coordinate data input device of an electrostatic capacity type, the protective sheet 3 provided with the adhesive layer 2 of an acrylic resin or the like on the back surface thereof is bonded adhesively to the sensing board 1. It is difficult to bond the protective sheet 3, which is a film of a thickness of about 0.2 mm, to the sensing board 1 so as to conform exactly to the irregular surface of the sensing board 1, and bubbles B are liable to be formed beside the x-electrodes 5 as shown in FIG. 3 because the protective sheet 3 is unable to conform exactly to the height of the x-electrodes 5. Air contained in the bubbles B sealed in the adhesive layer 2 affects capacitance. Therefore, when a finger is moved linearly on the protective sheet 3 of the conventional coordinate data input device in which many bubbles of indefinite shapes are liable to be formed on the surface of the sensing board 1, the linearity of coordinates varies from position to position, and the coordinate data input device cannot detect positions in a high accuracy.
Although the possibility of forming bubbles can be reduced by reducing the thickness of the protective sheet 3 because a thinner protective sheet can more easily conform to the irregularities. However, since the surface of the protective sheet of a reduced thickness, i.e., the touch surface, is liable to become irregular according to the underlying irregularities if the protective sheet 3 has a reduced thickness, capacity is liable to vary according to a position to which a touch is given with a finger and the operational facility of the coordinate data input device is deteriorated.